1. Field of the Invention
This invention relates to the field of damping devices for use in aircraft landing gear, particularly to dampers in which flow of viscous hydraulic or oleic fluid through an orifice of variable size dissipates impact energy associated with landing an aircraft.
2. Description of the Prior Art
Conventional dampers operate by throttling viscous fluid through orifices, usually formed through a piston separating a high pressure chamber from a low pressure chamber in a sealed cylinder in which the piston moves in response to relative movement of components of a landing gear to which the piston and cylinder are attached. Operation of the damper is enhanced if the areas of the orifices are changed while energy is dissipated.
Various means are employed to alter the size of the orifices, often in accordance with changes in pressure of the viscous fluid within the cylinder chambers. U.S. Pat. No. 4,428,567 describes a shock absorber in which two oil reservoirs, separated by a piston, communicate through a bore that is open permanently and through ports controlled by reed valves that open and close in response to the magnitude of differential pressure across the piston.
The magnitude of forces produced by springs actuated by relative displacement of a piston and a cylinder containing the piston are often used to open and close piston orifices. For example, in the shock absorber described in U.S. Pat. No. 3,365,101, a spring-biased valve plate closes control ports in a piston until a magnitude of pressure developed by the stroke of the piston becomes sufficient to open the control ports. In the hydro-pneumatic motor of U.S. Pat. No. 3,079,897, the size of a spring-loaded valve having a variable cross section in a piston head is responsive to flow resulting from relative motion between a cylinder and piston to vary the area of the valve opening and maintain constant the rate of flow. U.S. Pat. No. 3,865,356 describes an automotive shock absorber, in which piston throttling orifices are closed by a spring-loaded valve and are opened to permit fluid flow when an axial force reached a specified magnitude.
A shock absorber for an aircraft landing gear is described in U.S. Pat. No. 4,381,857 wherein impact energy is dissipated by viscous flow through several orifices and rebound energy is dissipated through a greater number of orifices, each orifice maintaining its original size.
However, the size of damper orifices can be made to vary in accordance with the position of the piston during the stroke of the damper. An example of this technique is described in U.S. Pat. No. 4,082,255. There the effective size of a orifice defined by an annular space between an aperture of fixed size in an orifice fitting supported on an orifice tube and a metering rod of variable diameter, changes as the rod passes through the fitting during the damper stroke. The outer diameter of the metering rod is shaped as required to produce the requisite variable area between the fitting and the rod.
Metering rod dampers have theoretically high operating efficiency due to their ability to vary the rate of fluid flow across the orifice formed between the metering rod and the aperture in the orifice fitting. However, such dampers present many disadvantages. Generally, the metering rod, orifice support tube and attachment fittings add weight to the damper. Functionally, the outer cylinder, piston rod, metering rod and orifice support tube are susceptible to bending deflections due to landing forces induced by side loads and drag loads applied to the end of the piston rod. This bending reduces substantially the service life of the parts affected by it. Bending deflection of the metering rod and orifice support tube causes eccentric movement of the rod in the orifice and effects contact between the rod and one side only of the orifice. The localized nature of the contact induces excessive wear of the orifice aperture, which adversely influences damping characteristics.
Fixed orifice dampers operate inherently at lower efficiency than variable orifice dampers. Bending developed as a result of drag and side landing loads is reacted between a bearing supporting the piston rod at the end of the cylinder and a piston head bearing located between the periphery of the piston and the inner surface of the cylinder. Axial distance between these reaction forces is small, particularly when the piston is fully extended from the cylinder end; therefore, the magnitude of reaction forces is high, and bearing pressures and wear are excessive.